A motor drive system is generally used which converts DC voltage into AC voltage by an inverter to drive and control an AC motor. In such a motor drive system, generally, motor current is controlled according to sinusoidal PWM (Pulse Width Modulation) control based on vector control for driving an AC motor with high efficiency.
However, in the sinusoidal PWM control method, a fundamental component of an output voltage of the inverter cannot be increased enough and voltage utilization is limited, so that it is difficult to obtain a high power in a high rotational speed region. In consideration of this point, employment of a modulation method that allows outputting a voltage with a fundamental component larger than the sinusoidal PWM control method has been proposed.
For example, Japanese Patent Laying-Open No. 2000-50689 (Patent Document 1) proposes that, in a control configuration in which, for output power improvement in a high speed range, rectangular wave voltage is applied to an AC motor to rotate and drive this AC motor (also referred to as “rectangular wave control method” hereinafter), torque control of AC electric motor is performed by controlling the phase of this rectangular wave voltage based on a deviation between a torque command value and actual torque.
Furthermore, such a configuration is disclosed in that a motor drive system additionally employing “overmodulation PWM control method” using an intermediate voltage waveform between the rectangular wave control method and the sinusoidal PWM control method is adopted to a hybrid automobile (for example, “Toyota's Motor Control Technology Achieving Balance Between Ecology and Power,” Nikkei Monozukuri, August 2004, p. 89-95). This motor drive system uses the three control methods, namely, the sinusoidal PWM control, the overmodulation PWM control and the rectangular wave control method, which are appropriately switched depending on the motor operation condition (typically, torque/revolutions).
In addition, such a configuration is disclosed in that, considering that when an output voltage of an inverter is saturated in motor current feedback control, a harmonic also superimposes on current flowing in a motor, motor current feedback control is performed after a harmonic component is removed from the detected motor current by a filter operation portion (for example, Japanese Patent Laying-Open No. 9-215398: Patent Document 2).
Here, in the modulation method in which a fundamental component of an inverter output voltage is larger than in the sinusoidal PWM control method, typically, in the rectangular wave control method and the overmodulation control method as described above, while output power can be improved in a mid-speed range and a high-speed range of an AC motor, control response is degraded as compared with the sinusoidal PWM control method, because the voltage waveform of the motor applied voltage is distorted.
This is because, in the rectangular wave control method, an operation amount is only a phase of motor applied voltage (rectangular wave voltage) and therefore the controllability is degraded as compared with the sinusoidal PWM control method which can employ both the amplitude and the phase of the applied voltage as the operation amount. In addition, a distortion component of motor current is greater because of a greater distortion in the motor applied voltage waveform. This requires filter processing similar to that in the aforementioned Patent Document 2 (for example, time constant: about a few tens of milliseconds), and also in this respect, a control delay occurs.
Furthermore, even in the overmodulation control method, a distortion component of motor current is greater similarly, and the controllability is degraded by the aforementioned filter processing, as compared with the sinusoidal PWM control method.
Therefore, in such a modulation method like the rectangular wave control method and the overmodulation control method in which a fundamental component of an inverter output voltage is larger than in the sinusoidal PWM control method, the control response of the motor applied voltage is delayed at a time of a sudden change of motor revolutions (which means revolutions per unit time and is synonymous with rotational speed. The same applies hereinafter), so that motor current is likely to be disturbed. In particular, if motor current is disturbed so as to diverge in the increasing direction, overcurrent/overvoltage is generated, which may lead to inconvenience such as system shutdown or equipment destruction. In addition, if the rating (withstand voltage, current capacity, and the like) of the system component is designed to excessively respond to the likelihood of overvoltage/overcurrent resulting from the degraded control response as described above, the manufacturing costs are increased.